of: thermal: Mark cooling-*-level properties optional

The cooling-{min,max}-level properties are marked as optional in
Documentation/devicetree/bindings/cpufreq/cpufreq-dt.txt and the usage
in various device tree matches this, i.e., some cooling device in the
device trees provide these properties while others do not.

Make the bindings in
Documentation/devicetree/bindings/thermal/thermal.txt consistent with
the cpufreq-dt bindings by marking the cooling-*-level properties as
optional.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Rob Herring <robh@kernel.org>